tor gets completed and it
doesn't check, if a callback is at all defined, which can lead to an Oops. Fix
these bugs and make ipu_update_channel_buffer() void, because it never fails.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
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